Handover (HO) means that a mobile station moves from a radio interface of one base station to a radio interface of another base station. Hereinafter, a handover procedure in a general IEEE 802.16e system will be described.
In an IEEE 802.16e network, a serving base station (SBS) may broadcast neighboring base station information through a neighboring advertisement (MOB_NBR-ADV) message to notify a mobile station (MS) of information (topology) on basic network configuration.
The neighboring advertisement (MOB_NBR-ADV) message includes system information on a serving base station and neighboring base stations, for example, preamble index, frequency, handover (HO) optimization capability information and downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information.
The DCD/UCD information includes information to be known by the mobile station to perform information, exchange through a downlink and an uplink. For example, examples of the information to be known by the mobile station include handover trigger information, medium access control (MAC) version of the base station, and media independent handover (MIH) capability information.
The general neighboring advertisement (MOB_NBR-ADV) message includes information only on neighboring base stations of the IEEE 802.16e type. Accordingly, neighboring base station information of another type except for the IEEE 802.16e type may be broadcasted to the mobile stations through a service identity information advertisement (SII-ADV) message. In this case, the mobile station may acquire information on heterogeneous network base stations by requesting the serving base station to transmit the SII-ADV message.
A handover procedure performed in the IEEE 802.16e network by the mobile station that has acquired neighboring base station information through the aforementioned method will be described in more detail.
The handover procedure in the general IEEE 802.16e network may include three procedures, i.e., a handover initiation & preparation procedure, a handover execution procedure and a handover completion procedure.
An example of the basic handover procedure as above will be described with reference to FIG. 1.
FIG. 1 is a diagram illustrating an example of a handover procedure that may be performed in the IEEE 802.16e system.
Referring to FIG. 1, the mobile station MS is registered with a serving base station SBS to transmit and receive data to and from the serving base station SBS (S101).
The serving base station SBS may periodically broadcast information on neighboring base stations existing within its coverage to mobile stations through a MOB_NBR-ADV message (S102).
The mobile station may start scanning for candidate handover base stations based on a handover (HO) trigger condition while performing communication with the serving base station. If a scanning value exceeds a handover condition, for example, predetermined hysteresis margin, the mobile station may transmit a handover request (MOB_MSHO-REQ) message to the serving base station to request the serving base station to perform the handover procedure (S103).
The serving base station that has received the handover request message may notify the candidate handover base stations included in the MOB_MSHO-REQ message, of handover request through a handover request (HO-REQ) message (S104).
For the mobile station, which has requested handover, the candidate handover base stations transfer handover related information to the serving base station through a handover response (HO-RSP) message (S105).
The serving base station transmits the handover related information acquired from the candidate handover base stations through the HO-RSP message to the mobile station through a handover response (MOB_BSHO-RSP) message. In this case, the MOB_BSHO-RSP message includes information required for handover, such as action time for performing handover, handover identifier (HO-ID), and a dedicated handover CDMA ranging code (S106).
The mobile station may select one target base station from the candidate handover base stations based on handover information included in the MOB_BSHO-RSP message received from the serving base station. As a result, the mobile station may try ranging by transmitting the CDMA code to the selected target base station (S107).
The target base station that has received the CDMA code may transmit success or failure of ranging and physical correction values to the mobile station through a ranging response (RNG-RSP) message (S108).
Next, the mobile station may transmit a ranging request (RNG-REQ) message for authentication to the target base station (S109).
The target base station that has received the ranging request message of the mobile station may provide system information, which may be used in the corresponding base station, such as connection identifier (CID), to the mobile station through a ranging response message (S110).
If the target base station transmits update information after successfully performing authentication of the mobile station, it may notify the serving base station of the mobile station of handover success or failure through a handover completion (HO-CMPT) message (S111).
Afterwards, the mobile station may transmit and receive information to and from the target base station that has performed handover (S112).
A handover procedure, which may be performed in the IEEE 802.16m system, is similar to the aforementioned handover procedure of the IEEE 802.16e system. However, titles of respective messages may be used differently as follows.
MOB_NBR-ADV→AAI_NBR-ADV: the corresponding message includes system information transmitted in a format of S-SFH not DCD/UCD.
MSHO-REG→AAI_HO-REQ
BSHO-RSP→AAI_HO-CMD
RNG-REQ (DCMA code)→Ranging preamble code
RNG-RSP (ranging status)→AAI_RNG-ACK (ranging status)
RNG-REQ (MAC message)→AAI_RNG-REQ
RNG-RSP→AAI_RNG-RSP: the corresponding message includes TSTID or STID, which is a station identifier, not CID.
Next, an idle mode will be described.
If the mobile station does not receive traffic from the base station for a certain time, it may be shifted to an idle mode for power saving. The mobile station shifted to the idle mode may receive a broadcast message (for example, paging message) from the base station for an available interval to determine whether it is to be shifted to a normal mode or maintained at the idle mode. Also, the mobile station in the idle mode may notify a paging controller of its location by performing location update.
The idle mode allows activity request related to handover and general operation requests to be removed, whereby the mobile station may be benefited. The idle mode may save power and operation resources used by the mobile station by restricting activities of the mobile station to perform scanning for a discrete period.
Also, the idle mode provides a simple and proper scheme that may notify the mobile station of pending downlink traffic, and removes a radio interface and network handover traffic from a mobile station of a non-active state, whereby the network and the base station may be benefited.
Paging means a function for identifying a location (for example, random base station or random relay station) of a corresponding mobile station when a call signal is generated in accordance with mobile communication. A plurality of base stations (BSs) that support an idle mode belong to a specific paging group to configure a paging region.
In order to increase a hit ratio of paging, the mobile station may perform a location update procedure. Location update is an operation of the mobile station, which is performed to increase a hit ratio during paging for the mobile station that has entered the idle mode, and means a process of reporting a new location or zone of the mobile station to the network when the mobile station moves to the new zone. This location update procedure may be performed by a method for exchanging a ranging request (RNG-REQ) message and a ranging response (RNG-RSP) message between the mobile station and the base station.
In the mean time, there exists a sleep mode, which is similar to the idle mode but does not return station identifier (basic CID in the IEEE 802.16e system and STID in the IEEE 802.16m system). A mobile station of an active state may shift to the sleep mode to minimize power consumption. If this mobile station moves to another base station (target base station), it performs a handover procedure including a handover ranging procedure that includes selecting any one of handover ranging code sets and transmitting the selected one to the base station.
Hereinafter, a deregistration with content retention mode (DCR mode) will be described.
The DCR mode means that a context of the mobile station is retained by a network entity for an effective period of a predetermined timer even though the mobile station is deregistered from the network. For the effective period of the predetermined timer, the network retains information required for quick network re-entry of the mobile station. When the mobile station enters the DCR mode, context retention identifier (CRID) is allocated to the mobile station. In order that the mobile station shifts from the DCR mode to the active mode, the mobile station may perform a ranging procedure by transmitting any one of handover ranging codes to the base station.
In the aforementioned procedures, the mobile stations having different states (active, sleep, idle, and DCR) from one another should transmit a ranging request (AAI_RNG-REQ) message to the corresponding base station to perform the ranging procedure for their respective purposes. In this case, each mobile station should be allocated with an uplink resource for transmitting the ranging request message from the corresponding base station. To this end, the mobile station first transmits a ranging code to the base station.
At this time, since the mobile station selects any one of same handover ranging code sets, the base station cannot identify a status of the mobile station that has transmitted the corresponding code and an intention of the mobile station. For this reason, the base station has to allocate an uplink resource of a fixed size to have the mobile station transmit a ranging request message. At this time, if the fixed size is smaller than a size required for transmission of parameters, the mobile station should additionally request the uplink resource to transmit the other parameters. By contrast, if the fixed size is greater than a size required for transmission of parameters, waste of resources waste occurs. Accordingly, the mobile station needs to efficiently notify the base station of its status while it is performing the ranging procedure.